Viewers may get to see, Lanie and Esposito's romance bloom.

Castle season 7 will air its midseason finale tonight, 8 December, which is a Christmas special episode titled Bad Santa.

In the last episode of 2014, Castle and Beckett investigate a mob doctor's murder mystery. The investigation will lead the mystery novelist to an old friend who has a strong mafia connection.

The doctor has been shot by a man dressed as Santa Clause and the newly married investigator couple are searching for clues at a strip club and later went to seek help from Rick's old mafia friend who appears visibly unhappy to know that his novelist friend is married to an investigating officer.

The official synopsis of the winter finale reads: "When an ER physician is gunned down on the street, Castle and Beckett soon discover that the victim was a mob doctor to a Mafia family run by an old friend of Castle's. Also, against the backdrop of Christmas, Esposito and Lanie reach a defining moment in their relationship."

The finale may also focus on Lanie and Esposito's budding romance and showrunner David Amman has teased that the latest favourite couple of the ABC hit series will share some sweet moments during the holiday season.

"Esposito follows him into a subway, and the guy sees that he's a cop, pulls a gun and holds the car hostage. Our people on the outside are trying to figure out: 'Who is this guy, really, because he clearly isn't who we thought he was'," Amman told TV Guide.

"It does make him reflect on his relationship with Lanie, and consider its importance in his life," he added.